Process and device for producing hot-rolled strip from silicon steel

ABSTRACT

A method and a combined casting/rolling installation for production/processing of high-quality hot-rolled strip into grain-oriented electrical steel strip at low cost, provides for: a) melting a steel having a chemical composition in wt % of Si 2 to 7%, C 0.01 to 0.1%, Mn&lt;0.3%, Cu 0.1 to 0.7%, Sn&lt;0.2%, S&lt;0.05%, Al&lt;0.09%, Cr&lt;0.3%, N&lt;0.02%, P&lt;0.1%, remainder Fe and impurities; b) casting a strand having a thickness of 25 to 150 mm on a continuous casting installation; c) rolling into a strip in up to 4 rolling passes directly after casting the strand, wherein at least in one rolling pass a true strain is &gt;30% or the total true strain of all passes is &gt;50%; d) heating the strip to a final temperature of 1050 to 1250° C., preferably 1100 to 1180° C.; e) finish rolling the strip in a second rolling train, then f) cooling and winding the strip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application of InternationalApplication No. PCT/EP2009/063245 filed Oct. 12, 2009, which designatesthe United States of America, and claims priority to AustrianApplication No. A1634/2008 filed Oct. 17, 2008. The contents of whichare hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a process and a device for producinghot-rolled strip from silicon-alloyed steels for further processing toform grain-oriented magnetic steel strip. The further processing of thehot strip is not the subject of this application; it takes place by heattreatment and cold-rolling.

BACKGROUND

Grain-oriented magnetic steel strip, for example for subsequentprocessing to form laminated magnetic steel sheet for transformers orelectrical machines, is distinguished by low specific remagnetizationlosses and a high magnetic permeability. As the consumption ofelectrical energy increases and ever higher demands are placed on theefficiency of electrical machines, there is a high demand forhigh-quality and inexpensive magnetic steel sheets.

The production of magnetic steel strip can be subdivided into thefollowing process steps: steel production, hot-strip production andcold-strip production, heat treatment and strip coating (see Fact Sheet401, “Elektroband und -blech” [Magnetic steel strip and sheet],Stahl-Informations-Zentrum, Dusseldorf, Edition 2005).

Combined casting/rolling installations are known to a person skilled inthe art for particularly economical production of high-quality hotstrips, for example for subsequent processing to form automotive steelsheets (see e.g. EP 1662011 A1).

WO 98/46802 A1 discloses a process for producing grain-oriented magneticsteel sheets, wherein either a) a specific steel alloy is melted and athin strand is cast therefrom in a continuous casting installation, thenthe strand is separated, the slabs are annealed, finish-rolled andcooled, and the hot strip is wound up; or b) a specific steel alloy ismelted and a thin strand is cast therefrom in a continuous castinginstallation, then the strand is finish-rolled and cooled, and the hotstrip is wound up.

After the working steps as per a) or b), the hot strip is substantiallyannealed, rolled to the final thickness in a cold-rolling mill train,decarburized and subjected to targeted secondary recrystallization. Themolten steel alloy contains what are known as growth inhibitors,specifically sulfides, carbides or nitrides of the elements Mn, Cu andAl, which prevent the grain growth of the microstructure present afterthe finish-rolling. Depending on the temperature, these precipitationsalso affect the recrystallization as early as during the deformation,and immediately thereafter, in a manner such that a microstructure canbe produced which, in further consequence, is suitable for producing amaterial with the desired grain properties.

The process according to the prior art for the production of hot-rolledstrip either consumes a large amount of energy, or results in losses inquality of the grain-oriented magnetic steel sheets which are furtherprocessed. The equalizing furnaces used for annealing the slabs areadditionally not very compact, which in turn increases the capital costsfor the overall installation.

SUMMARY

According to various embodiments a process and a combinedcasting/rolling installation of the type mentioned in the introductioncan be provided, with which high-quality hot-rolled strip for furtherprocessing to form grain-oriented magnetic steel strip with outstandingmagnetic, electrical and geometrical properties can be produced at lowcost.

According to an embodiment, a process for producing hot-rolled stripfrom silicon-alloyed steels on a combined casting/rolling installationfor further processing to form grain-oriented magnetic steel strip, maycomprise the following process steps in the sequence given: a) a steelhaving a chemical composition (in % by weight) of Si 2 to 7%, C 0.01 to0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%,N<0.02%, P<0.1%, remainder Fe and impurities is melted; b) a strandhaving a thickness of 25 to 150 mm is cast on a continuous castinginstallation; c) a strip is rolled in up to 4 roll passes immediatelyafter the strand has been cast, wherein at least in one roll pass adegree of deformation is >30% or the total degree of deformation of allthe passes is >50%; d) the strip is heated to a final temperature of1050 to 1250° C., preferably 1100 to 1180° C.; e) the strip isfinish-rolled in a second rolling mill train, and then f) the strip iscooled and coiled.

According to a further embodiment, the final temperature after the striphas been heated may be maintained for a duration t, where t>15 s,preferably >60 s. According to a further embodiment, the finaltemperature of the strip can be maintained in a continuous furnace.According to a further embodiment, the final temperature of the stripcan be maintained during winding-up and subsequent unwinding in acoiling furnace. According to a further embodiment, the strip can befinish-rolled in 2 to 6, preferably 3 to 5, roll passes in the secondrolling mill train. According to a further embodiment, the strip mayhave a final rolling temperature of 900 to 1050° C. after thefinish-rolling. According to a further embodiment, the strip can becooled to a coiling temperature of 300 to 600° C. by means of anintensive cooling step within 10 s, preferably within 6 s, after thefinish-rolling. According to a further embodiment, at the start of theintensive cooling step, the strip can be cooled at a cooling rate whichis twice as high, preferably three times as high, as the cooling rate atthe end of the cooling step. According to a further embodiment, the sumof the alloying elements Cu+Mn in the steel melt can be >0.35% byweight, preferably >0.55% by weight. According to a further embodiment,the sum of the alloying elements S+N in the steel melt can be >100 ppm,preferably >200 ppm. According to a further embodiment, the quotient ofthe alloying elements Cu/Mn in the steel melt can be >2.5, preferably>3.5.

According to another embodiment, a combined casting/rolling installationfor producing hot-rolled strip from silicon-alloyed steels for furtherprocessing to form grain-oriented magnetic steel strip, may comprise acontinuous casting installation, a first rolling mill train, a heatingdevice, a second rolling mill train, a cooling section and a winding-updevice, wherein the first rolling mill train is arranged directlydownstream of the continuous casting installation, and a coiling furnaceor a continuous furnace for introducing heat and/or maintaining thetemperature of the hot strip is located between the heating device andthe second rolling mill train.

According to a further embodiment of the installation, the continuouscasting installation may be in the form of a thin-slab continuouscasting installation. According to a further embodiment of theinstallation, the first rolling mill train may comprise up to fourrolling stands. According to a further embodiment of the installation,the second rolling mill train may comprise 2 to 6, preferably 3 to 5,rolling stands.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages and features will become apparent from the followingdescription of non-limiting exemplary embodiments, where reference ismade to the following figures:

FIG. 1 is a schematic illustration of a combined casting/rollinginstallation for the discontinuous production of hot-rolled strip forfurther processing to form grain-oriented steel sheets,

FIG. 2 is a schematic illustration of a combined casting/rollinginstallation for the continuous production of hot-rolled strip forfurther processing to form grain-oriented steel sheets.

DETAILED DESCRIPTION

A high-quality hot-rolled strip of this type is understood to mean a hotstrip in which the growth inhibitors are distributed in the hot strip infinely dispersed form and homogeneously. This object is achieved by aprocess in which the following process steps are carried out in thesequence given on a combined casting/rolling installation:

a) a steel having a chemical composition (in % by weight) of S±2 to 7%,C 0.01 to 0.1%, Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%,Cr<0.3%, N<0.02%, P<0.1%, remainder Fe and impurities is melted;

b) a strand having a thickness of 25 to 150 mm is cast on a continuouscasting installation;

c) a strip is formed by rolling in up to 4 roll passes immediately afterthe strand has been cast, wherein at least in one roll pass a degree ofdeformation is >30% or the total degree of deformation of all the passesis >50%;

d) the strip is heated to a final temperature of 1050 to 1250° C.,preferably 1100 to 1180° C.;

e) the strip is finish-rolled in a second rolling mill train, and then

f) the strip is cooled and coiled.

In this production process, the formation of homogeneously distributedgrowth inhibitors present in finely dispersed form, specificallysulfides, nitrides and carbides of the elements Mn, Cu, Al but also Cr,is promoted by the melting of a specific steel alloy (step a) and therolling of a strip, which follows immediately after a thin strand hasbeen cast (step b), with high degrees of deformation (step c) on a firstrolling mill train. The degree of deformation φ is defined asφ=h₀−h₁/h₀, where h₀ is the thickness before the deformation and h₁ isthe thickness of the strip or strand after one or more deformationsteps; in this application, the degree of deformation is given in %. Theheating of the strip (step d) has the effect that the furtherprecipitation of growth inhibitors is stopped, and precipitations whichhave already formed with given kinetics are dissolved again. When thetemperature is reduced again during finish-rolling on a second rollingmill train (step e) and the subsequent cooling of the strip (step f),further homogeneously distributed growth inhibitors present in finelydispersed form are formed. The production process can either be carriedout continuously, i.e. on the basis of a strand or an unseparated strip,or in discontinuous batch operation, i.e. on the basis of slabs.

In one embodiment of the production process, the final temperature afterthe strip has been heated is maintained for a duration t, where t>15 s,preferably >60 s. Owing to this measure, a relatively high proportion ofprecipitations which may already be present in coarse clusters in thestrip is dissolved. It is not expedient to maintain the temperature fora time t where t>90 s, since all the precipitations are already presentin dissolved form after this time.

In continuous operation, the final temperature of the strip isadvantageously maintained in a continuous furnace, which, by way ofexample, is in the form of a gas-fired furnace or of an inductionfurnace. This makes it possible to maintain the temperature of the stripin a particularly compact manner in continuous operation.

In discontinuous batch operation, the final temperature of the strip isadvantageously maintained by winding-up and unwinding in a coilingfurnace. This makes it possible to maintain the temperature of the stripin a particularly compact manner in discontinuous operation.

In one embodiment of the process, the strip is finish-rolled in 2 to 6,preferably in 3 to 5, roll passes on a second rolling mill train. Thismakes it possible to produce common strip thicknesses in a particularlyeconomical manner.

In the case of finish-rolling, it is expedient if the strip has a finalrolling temperature of 900 to 1050° C. after the finish-rolling. Thisensures that the strip is finish-rolled in a favorable temperaturerange.

A further embodiment consists in the fact that the strip is cooled to acoiling temperature of 300 to 600° C. by means of an intensive coolingstep within max. 10 s, preferably within max. 6 s, after thefinish-rolling.

A further embodiment variant of the process consists in the fact that,at the start of the intensive cooling step, the strip is cooled at acooling rate which is twice as high, preferably three times as high, asthe cooling rate at the end of the cooling step. This temperature regimeensures that the microstructure present after the finish-rolling is“frozen” as quickly as possible for the subsequent steps.

In terms of the formation of growth inhibitors, it is advantageous forthe sum of the alloying elements Cu+Mn in the steel melt to be >0.35% byweight, preferably >0.55% by weight. For the formation of a sufficientlyhigh number of growth inhibitors, it is advantageous for the sum of thealloying elements S+N in the steel melt to be >100 ppm, preferably >200ppm. An adequate quantity of Cu, Mn, S and N in the steel melt isadvantageous in order to make it possible for a sufficient quantity ofgrowth inhibitors to be precipitated into the hot strip.

The quotient of the alloying elements Cu/Mn in the steel melt isadvantageously >2.5, preferably >3.5. Since Cu sulfides have a smallersize and a lower precipitation temperature than Mn sulfides, and aretherefore to be preferred, it is advantageous if the steel melt containsmore Cu than Mn. Since, however, Mn has more affinity to S than Cu, a“surplus” of Cu has to be present, in order to make it possible to forma higher quantity of Cu sulfides than Mn sulfides.

According to a further embodiment, for continuous operation the firstrolling mill train is arranged directly downstream of the continuouscasting installation, and a continuous furnace for introducing heatand/or maintaining the temperature of the hot strip is located betweenthe heating device and the second rolling mill train. This configurationof the installation makes it possible to carry out the process accordingto various embodiments in a particularly economical manner with a highproduct quality, i.e. high production performance (continuousoperation), low energy costs (amount of energy used to heat the hotstrip is minimized) and low capital costs (compact installation).

One embodiment of the combined casting/rolling installation consists inthe fact that the continuous casting installation is in the form of athin-slab continuous casting installation. A further embodiment consistsin the fact that the first rolling mill train comprises up to fourrolling stands. A further embodiment consists in the fact that thesecond rolling mill train comprises 2 to 6, preferably 3 to 5, rollingstands. As a result of these measures, the capital costs for the firstrolling mill train and the second rolling mill train are kept low(common hot-strip thicknesses can be produced on a few rolling stands).

Exemplary Embodiment 1

FIG. 1 shows a combined casting/rolling installation 1 for producinghot-rolled strip from silicon-alloyed steels; the installation parts forthe further processing of the hot strip to form a grain-orientedmagnetic steel strip are not shown. The states, i.e. the temperaturesand thicknesses, of the strand or strip in the individual process stepsare shown in table I; the states are denoted as P1 to P15. In acontinuous casting installation 2 for producing thin slabs, a strand 3having a thickness of 90 mm is cast from a specific steel alloy,consisting (in % by weight) of Si 3.2%, C 0.08%, Mn 0.1%, Cu 0.3%, Sn0.08%, S 0.01%, Al 0.03%, Cr 0.1%, N 0.012%, P 0.05%, remainder Fe andimpurities. Immediately after full solidification (strand temperature1174° C., state P1), the strand 3 is subjected to a first rolling step,consisting of 2 roll passes, on a first rolling mill train 5. Here, theindividual degrees of deformation are in each case 53% and 52%, i.e.first a strip having a thickness of 42 mm (state P2) and then a striphaving a thickness of 20 mm (state P3) are rolled. The temperature ofthe strip after the first pass is 1171° C., and after the second pass is1086° C. This first rolling step promotes the formation of homogeneouslydistributed clusters of growth inhibitors present in finely dispersedform, specifically sulfides, nitrides and carbides of the elements Cu,Al, Mn and Cr, in the strip, as a result of which further grain growthis inhibited. After the first rolling step, a roller table is used toconvey the strip 4 to a heating device 6, in the form of an inductionfurnace, in which the incoming strip cooled to 944° C. (state P4) isheated to a final temperature of 1150° C. (state P5). Then, thetemperature of the strip is maintained in a coiling furnace 7(temperature on entry into the coiling furnace is 1134° C., state P6)for at least 30 s. The residence time of a strip region, the so-calledlocal residence time, is different depending on the strip position.Owing to the winding-up and unwinding of the strip, it is the case, forexample, that the head of the strip—present before the winding—remainsin the coiling furnace for a longer time than the end of the strip; inthis sense, the head of the strip present before the winding becomes theend of the strip, and vice versa. The heating of the strip 4 preventsprecipitation of growth inhibitors until the strip is finish-rolled in asecond rolling mill train 8; by maintaining the temperature for a timet, coarse clusters of growth inhibitors are dissolved, which are formedagain in finely distributed form when the temperature is again reducedduring the finish-rolling. After the near-net strip has been wound upand unwound in the coiling furnace 7, scale is removed from the strip bymeans of a descaling installation 12, as a result of which thetemperature of the strip drops from 1101° C. to 1070° C. (temperaturesbefore and after the descaling, states P7 and P8). Then, the strip isfinish-rolled to a hot-strip final thickness of 2.6 mm on a secondrolling mill train 8 in four roll passes (individual degrees ofdeformation 55, 53, 28 and 16%, i.e. strip thicknesses of 9.1, 4.3, 3.1and 2.6 mm, states P9 to P12). During these roll passes, the strip coolsdown from 1043° C., 1012° C. and 984° C. to a final rolling temperatureof 955° C. after the last roll pass. After the finish-rolling, the stripis cooled on a cooling section 9 within 3 s after the last pass in thesecond rolling mill train 8 from 932° C. (cooling section entry, stateP13) to a temperature of 560° C. at the exit from the cooling section(state P14). During the finish-rolling and cooling of the strip, theclusters of growth inhibitors present in the strand are precipitated infinely dispersed form, i.e. with a typical cluster size <60 nm. Afterthe hot strip has been cut by means of shears 10, the strip is wound upin a winding-up device 11; the winding temperature is 540° C. here(state P15). In subsequent production steps (not shown in more detail),the present hot strip is annealed, rolled to the final thickness in acold-rolling mill train, decarburized and subjected to targetedsecondary recrystallization.

Exemplary Embodiment 2

FIG. 2 shows a further combined casting/rolling installation for thecontinuous production of hot-rolled strip from silicon-alloyed steels;the installation parts for the further processing of the hot strip toform a grain-oriented magnetic steel strip are again not shown. Thestates P1 to P5 and P7 to P15 of the strand or strip in the individualprocess steps can be gathered from table I. Here, in turn, a specificsteel alloy (see exemplary embodiment 1 for the chemical composition) ismelted and a strand 3 is cast therefrom in a continuous castinginstallation 2 (state P1). Immediately after full solidification, thestrand is subjected to a first rolling step, consisting of 2 rollpasses, on a first rolling mill train 5 (states P2 and P3). Then, thestrip 4 is heated in a heating device 6, in the form of an inductionfurnace (states P4 and P5). The significant difference with respect toexemplary embodiment 1 is that the temperature of the strip 4 is thenmaintained for at least 15 s after heating in a continuous furnace 13,in the form of a gas-fired furnace; the local residence time in thecontinuous furnace is constant for all strip regions (head of the strip,foot of the strip). The further process steps (descaling P7 to P8,finish-rolling P9 to P12, cooling P13 to P14 and coiling P15) can begathered from exemplary embodiment 1.

TABLE I Thickness Temp. Location [mm] [° C.] P1 End of the combinedcasting/rolling 90 1174 installation P2 After the first pass in thefirst 42 1171 rolling mill train P3 After the second pass in the first20 1086 rolling mill train P4 Entry to the heating device 20 944 P5 Exitfrom the heating device 20 1150 P6 Entry to the coiling furnace 20 1134P7 Entry to the descaling installation 20 1101 P8 Exit from thedescaling installation 20 1070 P9 After the first pass in the second 9.11043 rolling mill train P10 After the second pass in the second 4.3 1012rolling mill train P11 After the third pass in the second 3.1 984rolling mill train P12 After the fourth pass in the second 2.6 955rolling mill train P13 Entry to the cooling section 2.6 932 P14 Exitfrom the cooling section 2.6 560 P15 In the winding-up device 2.6 540

LIST OF REFERENCE NUMERALS

-   1 Combined casting/rolling installation-   2 Continuous casting installation-   3 Strand-   4 Strip-   5 First rolling mill train-   6 Heating device-   7 Coiling furnace-   8 Second rolling mill train 9 Cooling section-   10 Shears-   11 Winding-up device-   12 Descaling installation-   13 Continuous furnace

1. A process for producing hot-rolled strip from silicon-alloyed steelson a combined casting/rolling installation for further processing toform grain-oriented magnetic steel strip, comprising the followingprocess steps in the sequence given: a) melting a steel having achemical composition (in % by weight) of Si 2 to 7%, C 0.01 to 0.1%,Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%,P<0.1%, remainder Fe and impurities; b) casting a strand having athickness of 25 to 150 mm on a continuous casting installation; c)rolling a strip in up to 4 roll passes immediately after the strand hasbeen cast, wherein at least in one roll pass a degree of deformationis >30% or the total degree of deformation of all the passes is >50%; d)heating the strip to a final temperature of 1050 to 1250° C. or 1100 to1180° C.; e) finish-rolling the strip in a second rolling mill train,and then f) cooling and coiling the strip.
 2. The process according toclaim 1, wherein the final temperature after the strip has been heatedis maintained for a duration t, where t>15 s or >60 s.
 3. The processaccording to claim 2, wherein the final temperature of the strip ismaintained in a continuous furnace.
 4. The process according to claim 1,wherein the final temperature of the strip is maintained duringwinding-up and subsequent unwinding in a coiling furnace.
 5. The processaccording to claim 1, wherein the strip is finish-rolled in 2 to 6 or 3to 5 roll passes in the second rolling mill train.
 6. The processaccording to claim 1, wherein the strip has a final rolling temperatureof 900 to 1050° C. after the finish-rolling.
 7. The process according toclaim 1, wherein the strip is cooled to a coiling temperature of 300 to600° C. by means of an intensive cooling step within 10 s or within 6 safter the finish-rolling.
 8. The process according to claim 7, wherein,at the start of the intensive cooling step, the strip is cooled at acooling rate which is twice as high or three times as high as thecooling rate at the end of the cooling step.
 9. The process according toclaim 1, wherein the sum of the alloying elements Cu+Mn in the steelmelt is >0.35% by weight or >0.55% by weight.
 10. The process accordingto claim 1, wherein the sum of the alloying elements S+N in the steelmelt is >100 ppm or >200 ppm.
 11. The process according to claim 1,wherein the quotient of the alloying elements Cu/Mn in the steel meltis >2.5 or >3.5.
 12. A combined casting/rolling installation forproducing hot-rolled strip from silicon-alloyed steels for furtherprocessing to form grain-oriented magnetic steel strip, comprising acontinuous casting installation, a first rolling mill train, a heatingdevice, a second rolling mill train, a cooling section and a winding updevice, wherein the first rolling mill train is arranged directlydownstream of the continuous casting installation, and a coiling furnaceor a continuous furnace for at least one of introducing heat andmaintaining the temperature of the hot strip is located between theheating device and the second rolling mill train.
 13. The installationaccording to claim 12, wherein the continuous casting installation is inthe form of a thin-slab continuous casting installation.
 14. Theinstallation according to claim 12, wherein the first rolling mill traincomprises up to four rolling stands.
 15. The installation according toclaim 12, wherein the second rolling mill train comprises 2 to 6 or 3 to5 rolling stands.
 16. A system for producing hot-rolled strip fromsilicon-alloyed steels and further processing to form grain-orientedmagnetic steel strip, comprising: a) means for melting a steel having achemical composition (in % by weight) of S±2 to 7%, C 0.01 to 0.1%,Mn<0.3%, Cu 0.1 to 0.7%, Sn<0.2%, S<0.05%, Al<0.09%, Cr<0.3%, N<0.02%,P<0.1%, remainder Fe and impurities; b) means for casting a strandhaving a thickness of 25 to 150 mm; c) means for rolling a strip in upto 4 roll passes immediately after the strand has been cast, wherein atleast in one roll pass a degree of deformation is >30% or the totaldegree of deformation of all the passes is >50%; d) means for heatingthe strip to a final temperature of 1050 to 1250° C. or 1100 to 1180°C.; e) means for finish-rolling the strip, and f) means for cooling andcoiling the strip.
 17. The system according to claim 16, wherein thesystem is configured to maintain the final temperature after the striphas been heated for a duration t, where t>15 s or >60 s.
 18. The systemaccording to claim 17, wherein the system is configured to maintain thefinal temperature of the strip in a continuous furnace.
 19. The systemaccording to claim 16, wherein the system is configured to maintain thefinal temperature of the strip during winding-up and subsequentunwinding in a coiling furnace.
 20. The system according to claim 16,wherein the system is configured to finish-roll the strip in 2 to 6 or 3to 5 roll passes in the second rolling mill train.